JP6599843B2 - Electrodes, membrane electrode composites, electrochemical cells and stacks - Google Patents

Description

  Embodiments described herein relate generally to an electrode, a membrane electrode assembly, and an electrochemical cell and stack using the same.

  In recent years, electrochemical cells have been actively studied. Among electrochemical cells, for example, a polymer electrolyte membrane (PEMEC) is excellent in responsiveness to renewable energy such as photovoltaic power generation, so that it can be used as a hydrogen generator for large-scale energy storage systems. Use is expected. In order to ensure sufficient durability and electrolytic properties, a platinum nanoparticle catalyst is generally used for the cathode of PEMEC, and a particulate iridium catalyst is generally used for the anode.

  One major issue for the widespread use of PEMEC is cost reduction by reducing the amount of noble metal catalyst used.

  In recent years, carrier-less catalyst units having a porous structure or a laminated structure including a void layer have been proposed as noble metal catalysts for electrochemical cells. The form of the catalyst contained in these catalyst units is in the form of sponge or nanosheet, and high durability has been reported. Moreover, since the porosity of the catalyst layer containing these catalyst units is high, it is advantageous for mass transfer that is indispensable for the electrode reaction of the electrochemical cell, and is considered to be a promising technique for reducing the amount of noble metals. However, when the technique is applied to PEMEC, the stability of the water electrolysis performance is still not sufficient, and there is a need for further improvement.

JP 2010-33759 A

  The problem to be solved by the present invention is to provide an electrode, a membrane electrode composite, an electrochemical cell, and a stack that can provide stable water electrolysis performance with a small amount of noble metal.

The electrode of the embodiment includes a base material and a catalyst layer provided on the base material and including a plurality of catalyst units having a porous structure, and the catalyst layer has a first degree of dispersion in the vicinity of the base material. A first catalyst layer in which the catalyst unit is dispersed; and a second catalyst layer provided above the first catalyst layer and in which the catalyst unit is dispersed at a second degree of dispersion. Unlike first dispersion degree, the catalyst unit of the second catalyst layer, the average distance between catalyst units 30nm or more, Ru der below 2000 nm.

  Moreover, the membrane electrode assembly of the embodiment includes the electrode of the present invention.

  Moreover, the electrochemical cell of embodiment comprises the membrane electrode assembly of this invention.

  The stack of the embodiment includes the electrochemical cell of the present invention.

1 is a cross-sectional view of a membrane electrode assembly (MEA) according to one embodiment. The figure and SEM photograph which show the electrode which concerns on one Embodiment. Explanatory drawing which defines the height of the catalyst unit which concerns on one Embodiment, and the distance between units. The figure which shows the measurement spot of the electrode which concerns on one Embodiment. The figure which shows schematically the preparation methods of the electrode which concerns on one Embodiment, and a membrane electrode assembly. Sectional drawing of the electrochemical cell which concerns on one Embodiment. FIG. 3 is a cross-sectional view of a stack according to an embodiment. Explanatory drawing which defines the catalyst layer thickness which concerns on one Embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the following description, the same members and the like are denoted by the same reference numerals, and the description of the members and the like once described is omitted as appropriate.

  FIG. 1 shows a sectional view of a membrane electrode assembly (MEA) according to an embodiment of the present invention.

  The MEA 1 is constituted by the first electrode 2 and the second electrode 3 and the electrolyte membrane 4 disposed therebetween.

  The first electrode 2 adjacent to the electrolyte membrane 4 has a first gas diffusion layer (base material) 6 and a catalyst layer 5 laminated in order from the top of the drawing. The second electrode 3 has a second gas diffusion layer (base material) 8 and a catalyst layer 7 laminated in order from the bottom of the drawing. Here, the catalyst layer 5 has a multilayer structure in which the first catalyst layer 5A in the vicinity of the substrate 6, the intermediate catalyst layer (intermediate layer) 5C and the second catalyst layer 5B in the vicinity of the electrolyte membrane 4 are stacked, and each catalyst layer Are porous structures each containing a number of catalyst units. The porous structure refers to a structure including a large number of pores existing between catalyst units or inside a catalyst unit. The shape of the holes is not particularly limited. Each catalyst unit has a porous structure or / and a laminated structure including a void layer. When the base material 6 is the bottom, the first catalyst layer 5A is at the top in contact with the base material 6, and the second catalyst layer 5B is above it. “Upper” includes both the case of being above the intermediate layer 5C and the case of being simply above the first catalyst layer 5A. That is, the first catalyst layer 5A exists between the substrate 6 and the second catalyst layer 5B. In addition, it is preferable to include one or multiple intermediate layers 5C between the first catalyst layer 5A and the second catalyst layer 5B. The substrate 6 is in physical contact with the first catalyst layer 5A. The second catalyst layer 5B is in physical contact with the intermediate layer 5C or the first catalyst layer 5A. Further, the second catalyst layer 5B is in physical contact with the electrolyte membrane 4.

  The feature of the present invention is that the catalyst units contained in the second catalyst layer 5B in the vicinity of the electrolyte membrane 4 are dispersed from the first catalyst layer 5A in the vicinity of the base material 6. As a result, high durability of the substrate 6 and the electrode can be maintained, and at the same time, an uneven interface is formed between the catalyst layer 5 and the electrolyte membrane 4 to obtain stable water electrolysis performance.

  Below, the catalyst layer 5 of this embodiment is demonstrated in detail. Although the catalyst layer 5 of the first electrode 2 will be described, the catalyst layer 7 of the second electrode 3 may have the same structure as the catalyst layer of the first electrode 2.

The catalyst layer 5 according to the present embodiment is composed of a plurality of carrierless catalyst units having a porous structure or a laminated structure including a void layer, and has a high catalyst specific surface area (10 to 40 m ² / g) and a high porosity ( 70 vol% or more) and excellent durability. Support-less means that no support is used for the catalyst constituting the catalyst layer. One feature of the catalyst layer is that the catalyst in the catalyst unit is substantially continuous, and the catalyst units are often partially bonded to each other, so that the catalyst layer 5 is structurally very stable. It has become a state. A conventional catalyst layer constituted by a catalyst or catalyst particles supported by a carrier has a porosity of less than 70 vol% (volume%) and has insufficient structural stability. Conventional PEMEC anodes use iridium black or iridium oxide black and are carrier-free, but often have a particle diameter of more than a dozen nm. Mass transport essential for electrode reaction strongly depends on the porosity of the catalyst layer 5, and the electrode reaction area is proportional to the specific surface area of the catalyst. Therefore, a large amount of catalyst is required to achieve sufficient water electrolysis efficiency and durability. in use. In addition, the diameter in embodiment represents a circumscribed circle diameter.

  When the catalyst unit included in the catalyst layer 5 according to the present embodiment is a porous body, 80% or more (unit ratio) of the catalysts included in the catalyst unit have a diameter of 2 nm or more and 8 nm or less. It is an aggregate in which catalyst particles and fine pores having a diameter of 1 nm or more and 5 nm or less are mixed, and can be said to be a sponge. When the catalyst unit has a laminated structure including a void layer, the catalyst unit has a laminated structure mainly including a plurality of nanosheet catalysts and a void layer between the nanosheet catalysts. Depending on the composition of the catalyst, the nanosheet-like catalyst may be a continuous film, a porous film including pores having a diameter of 2 nm to 50 nm, or a continuous aggregation sheet composed of particles having a diameter of 2 nm to 8 nm. The first catalyst layer 5A preferably has a catalyst structure in which sheet-like structures extending in a method perpendicular to the stacking direction are stacked. The second catalyst layer 5B preferably has a catalyst structure in which columnar structures extending in the stacking direction are bundled. The second catalyst layer 5B having such a columnar structure has a structure extending from the first catalyst layer 5A in the direction of the electrolyte membrane 4, and physically connects the intermediate layer 5C or the first catalyst layer 5A and the electrolyte membrane 4. Preferably it is.

The porosity of the catalyst layer 5 according to this embodiment is preferably 50 vol% or more and 95 vol% or less. The porosity of the catalyst layer 5 is more preferably 70 vol% or more and 95 vol% or less from the viewpoint of structural stability. This is because, if the porosity of the catalyst layer 5 is within this range, the substance can be sufficiently moved without reducing the utilization efficiency of the noble metal. About the diameter of the catalyst in a catalyst unit, when it is a crystal body, a crystal grain diameter is calculated | required by X-ray diffraction analysis. The crystal diameter of platinum or iridium oxide is often 2 to 20 nm.

The catalyst layer 5 of the present embodiment has a catalyst unit with a different degree of dispersion near the base material 6 and the electrolyte membrane 4, has a catalyst unit with a low degree of dispersion near the base material 6, and has a degree of dispersion near the electrolyte membrane 4. It is characterized by comprising a high catalyst unit.

  The degree of dispersion indicates the distance between the catalyst units. When the degree of dispersion is high, the distance between the catalyst units is far away. When the degree of dispersion is low, the distance between the catalyst units is It is narrow. This degree of dispersion is an average distance between the catalyst units in a direction parallel to the surface of the substrate 6 facing the catalyst layer 2. When the difference between the average distance between the catalyst units of the first catalyst layer 5A and the average distance between the catalyst units of the second catalyst layer 5B is 10 nm or more, the first degree of dispersion of the first catalyst layer 5A and the second catalyst layer 5B The second degree of dispersion is different.

  The degree of dispersion of the catalyst unit existing in the vicinity of the substrate 6 is important for the durability of the electrode, particularly the anode of the PEMEC cell. The degree of dispersion of the catalyst unit in the vicinity of the base 6 is preferably a catalyst unit having a low degree of dispersion as in the present embodiment in terms of the life and durability of the base 6 and the entire electrode.

  By increasing the degree of dispersion of the catalyst units of the second catalyst layer 5B in the vicinity of the electrolyte membrane 4, each catalyst unit deeply penetrates into the electrolyte membrane 4 when the MEA is manufactured, and the second catalyst layer 5B and the electrolyte membrane 4 A stable water electrolysis performance can be obtained by forming a concavo-convex interface between them and increasing the contact area between the catalyst layer 5 and the electrolyte membrane 4. It is desirable that the catalyst layer 7 of the second electrode 3 has the same configuration as the catalyst layer 5 described above.

  As shown in the enlarged view of FIG. 1, when the thickness of the entire catalyst layer 5 is 1, regions having different degrees of catalyst unit dispersion have a thickness of at least 0 from the vicinity of the interface between the electrolyte membrane 4 and the catalyst layer 5, for example, the interface. The region up to .1 is preferably a region having a different degree of dispersion (for example, a region having a high degree of dispersion). An electrode having high durability can be obtained due to the high degree of dispersion of the catalyst in this region. If there is a region with a high degree of dispersion from the interface to a region thicker than the region having a thickness of 0.4, it is not preferable that the durability is lowered. Therefore, it is preferable that the region having a high degree of dispersion exists in a region from the interface to a thickness of 0.4. Such a region having a high degree of dispersion is preferably composed of the second catalyst layer 5B. Further, it is desirable that the vicinity of the interface between the base material 6 and the catalyst layer 5, for example, a region having a thickness of at least 0.1 from the interface is a region having a different degree of dispersion (for example, a region having a low degree of dispersion). Even if such a region is thick, it contributes to high durability and high catalytic ability, and therefore it is preferable that a region having a low degree of dispersion exists in a region having a thickness from the interface to 0.9. Such a region having a low degree of dispersion is preferably composed of the first catalyst layer 5A.

  It cannot be denied that the present catalyst layer 5 has an intermediate layer between catalyst layers having different degrees of dispersion.

  The degree of dispersion of the catalyst unit of the intermediate layer 5C in the intermediate region is not particularly limited. For example, the degree of dispersion is preferably lower than that of the second catalyst layer 5B. Further, the intermediate region or the intermediate layer 5C has a region or a structure in which the degree of dispersion gradually changes from a region having a high degree of dispersion toward a region having a low degree of dispersion from the second catalyst layer 5B side toward the first catalyst layer 5A side. Also good.

  FIG. 2 shows a cross-sectional image of an electrode according to an embodiment of the present invention and an SEM photograph of a related portion. FIG. 2A is an image diagram of the catalyst layer 5 in which the base material 6, the first catalyst layer 5A, and the second catalyst layer 5B are laminated in this order. FIG. 2B shows a cross-sectional SEM photograph of the catalyst unit of the second catalyst layer 5B having a high degree of dispersion near the electrolyte membrane indicated by 5B in FIG. On the other hand, FIG. 2 (C) shows a cross-sectional SEM photograph of the catalyst unit of the first catalyst layer 5A having a low degree of dispersion near the substrate indicated by 5A in FIG. FIG. 2D shows a cross-sectional SEM photograph of the catalyst unit of the second catalyst layer 5B having a high degree of dispersion. FIG. 2E shows a cross-sectional SEM photograph of the catalyst unit of the first catalyst layer 5A having a low degree of dispersion. 2B and 2C are images of a cross section perpendicular to the main surface of the substrate 6. 2D and 2E are images of a cross section in a direction perpendicular to the cross sections of FIGS. 2B and 2C. There is a difference in the degree of dispersion of the catalyst units at each location. The catalyst area and the other void area are discriminated from the contrast of the SEM photograph. A white area | region is a catalyst and a black area | region represents the space | gap. When obtaining the distance between the catalyst units, an SEM photograph taken at a magnification of 100,000 is used. Further, the catalyst units existing in the depth direction from the cross-section are determined from the focus and shading, and are excluded from the objects for which the distance between the catalyst units is obtained.

  An embodiment opposite to the present embodiment is also conceivable in which the catalyst layer 5 has a catalyst unit with a high degree of dispersion near the base material 6 and a catalyst unit with a low degree of dispersion near the electrolyte membrane 4 or in the surface region.

  In this case, since the degree of dispersion of the catalyst unit in the vicinity of the substrate 6 is high, there is an advantage that water supply to the electrode becomes smooth and there is no waste in movement of ions to the electrolyte membrane 4. However, if the degree of dispersion of the catalyst units in the vicinity of the base material 6 is high and there is a large gap between the catalyst units, there is a high possibility that the electrodes will deteriorate. Although the degradation mechanism has not yet been fully elucidated, it is thought that the base material is affected by liquids and protons.

  In this embodiment, the degree of dispersion of the catalyst units is defined using the average distance between the catalyst units.

  It is desirable that the average distance between the catalyst units of the second catalyst layer 5B in the vicinity of the electrolyte membrane 4 or the surface region is 30 nm or more and 2000 nm or less, and the average height of the catalyst units is 30 nm or more and 1000 nm or less. The average distance between the catalyst units of the first catalyst layer 5A in the vicinity of the substrate 6 is preferably 0 nm or more and less than 30 nm.

  When the average distance between the catalyst units of the second catalyst layer 5B is less than 30 nm, or the average height of the catalyst units is less than 30 nm, the stability of characteristics is insufficient. In addition, when the average distance between the catalyst units of the second catalyst layer 5B exceeds 2000 nm, or the average height of the catalyst units exceeds 1000 nm, the improvement of the stability effect is small and the production cost is often high. From the same viewpoint, it is more desirable that the average distance between the catalyst units of the second catalyst layer 5B is 100 nm or more and 500 nm or less, or the average height of the catalyst units is 50 nm or more and 800 nm or less. In addition, when the average distance between the catalyst units of the first catalyst layer 5A is 20 nm or more, the stability of characteristics is insufficient. If the distance in the thickness direction of the catalyst layer 5 between the catalyst units of the first catalyst layer 5A is less than 5 nm, the catalyst performance is reduced because the catalyst is too dense. From this viewpoint, it is desirable that the average distance of the catalyst units of the first catalyst layer 5A is 0 nm or more and 20 nm or less. Note that there are holes or openings having a size of 5 μm or more on the surface of the base material in contact with the catalyst unit such as a metal mesh, and there are also dispersions of the catalyst unit due to them, but the dispersity and meaning of this embodiment are different. When the distance between the catalyst units is 3 μm or more, it is excluded from the dispersion degree measurement.

  In this embodiment, in order to quantitatively evaluate the height of the catalyst unit or the distance between the catalyst units, the following definitions are made. A cross-sectional SEM photograph of the electrode is taken, and the electrolyte membrane 4 is distinguished from the vicinity of the electrolyte membrane 4 and the vicinity of the substrate 6 from the structural features of the catalyst layer 5 described above on the SEM photograph, and as shown in the image diagram of FIG. The interface between the catalyst unit in the vicinity and the other catalyst units is taken as a reference line 62. The height of the catalyst unit is a length in a direction perpendicular to the reference line, and the maximum distance 63 among the distances from the respective surface points of the respective catalyst units to the reference line 62 is defined as the height of the catalyst unit.

  Regarding the distance between the catalyst units, an intermediate line is obtained on the surface of each catalyst unit at half the height of the catalyst unit, and the shortest distance between the intermediate lines of adjacent catalyst units on the SEM photograph is determined as the catalyst unit. The distance 61 is defined. Taking the enlarged view of FIG. 1 as an example, when the thickness of the highly dispersed catalyst unit is, for example, 0.1 of the catalyst layer thickness, as in the second catalyst layer 5B, the catalyst from the surface of each catalyst unit. An intermediate line is obtained at a position where the layer thickness is 0.05, and the shortest distance between the intermediate lines of adjacent catalyst units on the SEM photograph is defined as an inter-unit distance 61.

When all of the catalyst units in the SEM photograph are separated, the distance between the catalyst units can be simply obtained. The catalyst unit in the SEM photograph may include one having a contact surface with an adjacent catalyst unit. Whether a catalyst unit having a contact surface is regarded as one catalyst unit or a plurality of catalyst units is determined under the following conditions. While in SEM photo side length of having a contact surface of the catalyst unit (SEM photograph on the left) and L _A. The length of the side having a contact surface of the catalytic unit of the other in SEM photograph (SEM photograph on the right) and L _B. The length of the side of the contact surface to L _0. When L _A , L _B and L ₀ satisfy 2 * L ₀ / (L _A + L _B )> 0.7, a plurality of adjacent catalyst units having contact surfaces are treated as a single catalyst unit. Whether the adjacent catalyst unit is a separated unit is determined from the left side to the right side of the SEM photograph. Three or more catalyst units may be considered as a single catalyst unit. In addition, when it is determined as a plurality of catalyst units and the distance 61 between the catalyst units measured by the method in the previous paragraph is zero, the distance between the catalyst units is set to zero.

  The image diagram of FIG. 3 assumes the second catalyst layer 5B. The method for obtaining the distance between the catalyst units of the first catalyst layer 5A is the same as described below. In the description of the method of obtaining the average distance between the catalyst units of the second catalyst layer 5B and the method of obtaining the average distance between the catalyst units of the first catalyst layer 5A, the common method is to obtain the average distance between the first catalyst units. Omitted. When the first catalyst layer 5A has a structure in which planar catalysts are stacked, for example, the structure shown in the SEM photograph of FIG. 8 is observed. When the total thickness of the catalyst layer 5 is 1, the catalyst unit existing at a position corresponding to the thickness of 0, 1 from the surface of the substrate 6 toward the second catalyst layer 5B or the thickness of 0, 1 The distance of the nearest catalyst unit from the position is the distance between the catalyst units of the first catalyst layer 5A. Similarly, when it is determined whether a catalyst unit having a contact surface is regarded as one catalyst unit or a plurality of catalyst units, many units are regarded as a single unit. The distance between the catalyst units separated in the stacking direction is not included in the distance between the catalyst units when measuring the degree of dispersion. When all the catalysts are stuck together, the distance between units is zero. A catalyst layer with zero inter-unit distance is the ultimate low dispersion.

  Here, the average distance between the catalyst units of the first catalyst layer 5A is D1, and the second average distance between the units is D2. D1 and D2 preferably satisfy 0.00 ≦ D1 / D2 ≦ 0.01. When such conditions are satisfied, the catalyst performance is improved because of excellent gas diffusibility as well as water. A more preferable range is 0.000 ≦ D1 / D2 ≦ 0.006.

Regarding the average height of the catalyst units or the average distance between the catalyst units, nine spots are designated in the surface of the catalyst layer 5 shown in FIG. 4, and the height of the catalyst unit and the catalyst in the nine spots are designated. The average value of the distance between units was defined as the average height of the electrodes and the unit average distance. Each spot is square and has an area of at least 5 mm ² . When the electrode length 80 and the electrode width 82 are L and W, respectively, the distance 81 is W / 10, and the distance 83 is L / 10.

The predetermined catalyst material employed in the catalyst layer 5 of the present embodiment can be appropriately selected according to the electrode reaction. From the viewpoint of catalytic activity and durability, a noble metal or noble metal oxide catalyst is desirable. For example, when used as an anode of PEMEC, a catalyst having a composition represented by A _u M _v X _1- uv is desirable. Here, 0.5 ≦ u + v <1.0, u is 0.5 ≦ u <1.0, v is 0 ≦ v <0.5, and element A is Ir, Pt, Ru , Rh, Os, Pd and Au, at least one selected from the group consisting of noble metal elements, and the element M is a group consisting of Ta, W, Hf, Si, Mo, Ti, Zr, Nb, V, Cr and Sn The element X is at least one selected from the group consisting of Co, Ni, Fe, Mn, Al and Zn. The aforementioned materials can be used as metals or oxides, and may be composite oxides or mixed oxides containing two or more metals. The crystal structure of the catalyst is not particularly limited, but the durability of the crystal is often better than that of the amorphous. The intermediate layer is also preferably a layer containing crystals and amorphous of the catalyst material of the catalyst layer.

  When the electrode of this embodiment is applied to an anode for water electrolysis, 50 vol% or more of the catalyst layer in the vicinity of the electrolyte membrane 4 is platinum, and 60 vol% or more of the catalyst layer 5 in the vicinity of the substrate 6 is mainly composed of iridium oxide. It is desirable to be an oxide catalyst. The configuration, elements, and oxide distribution of the catalyst layer 5 can be confirmed by elemental mapping using a transmission electron microscope (TEM). When the catalyst layer 5 in the vicinity of the substrate 6 is composed of a catalyst unit having a laminated structure including a void layer and a catalyst layer, a noble metal layer such as platinum is disposed between the oxide catalyst layers such as iridium oxide inside the catalyst unit. However, the use of a mixed layer of iridium oxide and platinum may further improve the water electrolysis stability. The mechanism is unknown, but it may have improved proton conduction or electrical conduction.

  The electrode substrate 6 is generally required to be porous and conductive. When used as an anode of a water electrolysis cell, a titanium material is generally employed to ensure durability. Although it does not specifically limit about the form of a base material, The close which consists of a titanium mesh, a titanium fiber, a titanium sintered compact, etc. are mentioned. The water electrolysis characteristics may be improved by adjusting the aperture ratio of the porous substrate, particularly the pore structure of the portion in contact with the catalyst layer, or by surface treatment of the substrate such as blast treatment. This is probably because water supply to the catalyst layer, discharge of the electrode reaction product, and the like became smooth, and the electrode reaction in the catalyst layer was promoted. You may attach another coating layer on a base material. In some cases, the durability of the electrode is greatly improved by the dense conductive layer. Although a coating layer is not specifically limited, Ceramic materials, such as a metal material, an oxide, and nitride, carbon, etc. can be used.

  When a metal material such as titanium is used as the base material, the durability of the base material 6 can be improved by introducing the above-described coating as a base material protective layer on the surface of the base material 6. The base material protective layer can be further improved in durability by forming a multilayer structure or an inclined structure made of different materials. When titanium is used for the substrate, an oxide layer containing iridium having a thickness of 10 nm or more is particularly effective as the substrate protective layer. It is considered that a dense complex oxide layer of iridium and titanium was formed on the base material protective layer.

  The electrolyte membrane 4 is often required to have ionic conductivity. Examples of the electrolyte membrane having proton conductivity include fluorine resins having a sulfonic acid group (for example, Nafion (manufactured by DuPont), Flemion (manufactured by Asahi Kasei Co., Ltd.), and Ashiblek (manufactured by Asahi Glass Co., Ltd.)), tungstic acid and phosphorus Inorganic materials such as tungstic acid can be used.

  The thickness of the electrolyte membrane 4 can be appropriately determined in consideration of the characteristics of the MEA. From the viewpoint of strength, dissolution resistance and MEA output characteristics, the thickness of the electrolyte membrane is preferably 10 μm or more and 200 μm or less.

  A method for manufacturing the electrode and MEA according to the present embodiment will be briefly described (FIG. 5).

  Regarding the electrode, first, a material containing a catalyst material and a pore former material are sputtered on a base material by sputtering or vapor deposition to prepare a catalyst layer precursor. Next, the pore-forming agent is removed to obtain an electrode. Basically, in the case of a laminated structure precursor including a void layer, a material including a catalyst material and a pore former material are sequentially formed on a base by sputtering or vapor deposition. In the case of a porous structure precursor, the catalyst material and the pore former material can be simultaneously formed on the substrate by sputtering or vapor deposition. A catalyst layer having a catalyst unit with a high degree of dispersion can be produced by adjusting the ratio between the catalyst material and the pore-forming material, the pore-forming material removal process, and the like. Although it depends on the composition of the catalyst, the production is often facilitated by increasing the proportion of the pore-forming material. Alternatively, sputtering or vapor deposition may be performed by placing a porous mask on a catalyst layer having a catalyst unit having a low degree of dispersion. A catalyst layer having a highly dispersed catalyst unit can be produced by adjusting the porosity and pore size of the porous mask.

In the case of producing a catalyst layer made of an oxide such as an iridium oxide catalyst, a reactive sputtering method such as adding oxygen gas to the chamber during sputtering or vapor deposition can be employed. By optimizing parameters such as partial pressure of oxygen gas, power supply power during sputtering or vapor deposition, and substrate temperature, it is possible to greatly improve the durability of the electrode and the characteristics of the electrochemical cell.
Further, by performing post-treatment such as heat treatment after removing the pore-forming agent, it is possible to improve the activity and durability of the catalyst and improve the structure of the catalyst layer.

  When the catalyst layer of the present invention is directly formed on a substrate such as titanium, a dense interface layer can be formed between the catalyst layer and the substrate, and the deterioration of the substrate is greatly suppressed as a substrate protective layer. It is possible. Accordingly, the electrode of the present invention has excellent durability.

  The MEA according to the embodiment is manufactured by using the above-described catalyst layer as at least one of the first and second catalyst layers 5 and 7 in FIG. In the formation of the uneven interface between the catalyst layer and the electrolyte membrane of the MEA of this embodiment, the joining process between the catalyst layer and the electrolyte membrane is important. By this joining process, the amount of catalyst unit biting into the electrolyte membrane, the biting distribution and uniformity of each catalyst unit in the entire catalyst layer can be controlled.

  In general, the catalyst layer and the electrolyte membrane are joined by heating and pressurizing to produce a membrane electrode assembly. Both electrodes of the membrane electrode assembly are laminated and bonded as shown in FIG. 1 with the electrolyte membrane 4 sandwiched between electrodes including the catalyst layer 5 and the catalyst layer 7 when the base material of the catalyst layer is a gas diffusion layer. To obtain MEA1. When the base material of the catalyst layer is a transfer substrate, heat and pressure are applied to transfer the catalyst layer from the transfer substrate to the electrolyte membrane, then a gas diffusion layer is placed on the catalyst layer and bonded to the counter electrode Thus, the MEA 1 can be manufactured.

Joining of each member as described above is generally performed using a hot press machine. The pressing temperature is higher than the glass transition temperature of the polymer electrolyte used as the binder in the electrodes 2, 3 and the electrolyte membrane 4, and is generally 100 ° C. or higher and 300 ° C. or lower. Although the pressing pressure and the pressing time depend on the hardness of the electrodes 2 and 3, generally, the pressure is 5 kg / cm ² or more and 200 kg / cm ² or less, and the time is 5 seconds to 20 minutes. In order to precisely control the amount of biting in the catalyst unit, it is important to adjust the parameters of the hot press machine. In the present invention, in order to obtain the optimum biting amount, distribution and uniformity, the local temperature or the local pressure of the hot press machine is controlled in accordance with the physical properties and flatness of the substrate with the catalyst layer.

  Note that the following process may be employed for joining the catalyst layer and the electrolyte membrane. An electrolyte membrane is formed on a substrate with a catalyst layer, and a counter electrode catalyst layer is attached thereon. If a base material is a gas diffusion layer, it can be used as it is as MEA1. If the substrate is a transfer substrate, the gas diffusion layer is replaced before use as the MEA 1. In this case, the bite of the catalyst unit can be controlled by the concentration of the solvent for forming the electrolyte membrane, the configuration and formation temperature, the time, and the like.

  As described above, the MEA 1 according to the embodiment has high water electrolysis stability because an optimal electrode-electrolyte membrane interface is used.

  It is also possible to control the bite of the catalyst unit into the electrolyte membrane by cell assembly, for example, the clamping pressure perpendicular to the MEA.

The configuration and manufacturing method of the electrochemical cell according to this embodiment will be briefly described with reference to FIG. The electrochemical cell 10 includes the MEA 1 according to the embodiment, gaskets 11 and 12, current collecting plates 13 and 14, and fastening plates 15 and 16. MEA 1 is supported by gaskets 11 and 12. The MEA 1 supported by the gaskets 11 and 12 is sandwiched between current collector plates 13 and 14. A current collecting plate 13 exists between the MEA 1 and the fastening plate 15. A current collecting plate 14 exists between the MEA 1 and the fastening plate 16. Current collector plates 13 and 14 and fastening plates 15 and 16 are attached to both sides of MEA 1 of the embodiment via gaskets 11 and 12, and electrochemical cell 10 is manufactured by fastening with appropriate pressure.

The configuration and manufacturing method of the stack according to this embodiment will be briefly described with reference to FIG. The stack 20 has a configuration in which a plurality of electrochemical cells 10 are connected in series. A stack is prepared by attaching clamping plates 21 and 22 to both ends of the electrochemical cell and clamping them with an appropriate pressure.
(Example)

  Hereinafter, examples and comparative examples will be described.

  Tables 1 and 2 summarize the observation results of the electrodes of Examples 1 to 7 and Comparative Examples 1 to 3, the evaluation results of the PEMEC cell, and the like.

<Production of electrode>
(PEMEC standard cathode fabrication)
As a base material, carbon paper Toray 060 (manufactured by Toray Industries, Inc.) having a carbon layer with a thickness of 1 μm or more and 50 μm or less was prepared. On this base material, a catalyst layer composed of a catalyst unit having a laminated structure including a void layer is formed by sputtering to have a Pt catalyst loading density of 0.1 mg / cm ² and has a porous catalyst layer. An electrode was obtained. This electrode was used as a standard cathode in Examples 1 to 7 and Comparative Examples 1 to 3.
(PEMEC anode preparation Examples 1-7, Comparative Examples 1-3)

A titanium mesh base material subjected to surface treatment was prepared as a base material.
An electrode having a catalyst layer was obtained on this substrate by sputtering. In sputtering, the process was adjusted so that the shape of the catalyst unit and the thickness of the catalyst layer were the values shown in Tables 1 and 2 above. In some cases, heat treatment was performed at a temperature of 300 to 500 ° C. for 30 minutes to 4 hours.

The porosity, the catalyst unit average height, and the average distance between the catalyst units of the various electrodes produced were evaluated as follows. First, 9-spot samples were cut out from the electrodes obtained in Examples 1 to 7 and Comparative Examples 1 to 3 (FIG. 4). Next, the sample was cut from the center of the 9-spot sample to prepare a sample for SEM observation. Nine samples of each electrode were observed by SEM at three locations. 2000 to 300,000 times SEM images were obtained, and the catalyst and the pore were distinguished from the contrast. In the present invention, the catalyst layer thickness was measured from an SEM photograph. Specifically, using the SEM images of 2 to 50,000 times obtained at each of the above locations, as shown in FIG. 8, the thickness of the catalyst layer at three locations in each field of view is measured, and the average of the measured values of the above samples. The value was calculated as the average catalyst layer thickness of this electrode. Based on the catalyst layer thickness thus obtained, the noble metal catalyst thickness was calculated from the amount (g) of the noble metal catalyst and the density of each electrode, and the porosity of the catalyst layer was determined (1-noble metal catalyst thickness / catalyst layer thickness). . The catalyst density can be determined theoretically or experimentally in the absence of literature values for the catalyst. Note that the density of the platinum catalyst and iridium oxide catalysts used in Examples 1-7 and Comparative Examples 1 to 3 was employed 21.45 g / cm ³ and 11.66 g / cm ³ of literature values.

  SEM observation was performed on the catalyst layers of Examples 1 to 7 and Comparative Examples 1 and 3, and both included a laminated unit including a porous structure or a void layer, and the porosity of the catalyst layer was 80 It was confirmed to be -95 vol%.

  The XRD pattern of the electrode was obtained by X-ray diffraction (XRD) for the size of the crystal grains of the catalyst, the half width of the characteristic peak attributed to the catalyst was measured, and determined by the Scherrer method.

  The sizes of the catalyst crystal grains of Examples 1 to 7 and Comparative Examples 1 to 3 are all 3 to 20 nm.

  The average height of the catalyst units in the vicinity of the electrolyte membrane and the average distance between the catalyst units were determined by the same method as described in the preceding sentence. Tables 1 and 2 summarize the average value of the measured values of all the spots of each electrode as the average value of this electrode. In addition, it was confirmed that the catalyst units near the base material of each electrode are connected with almost no gap.

<Production of MEA for PEMEC>
4 cm × 4 cm square sections were cut from the PEMEC standard cathode and various PEMEC anodes. A standard cathode, an electrolyte membrane (Nafion 117 (manufactured by DuPont)), and various anodes were combined and thermocompression bonded to obtain various PEMEC MEAs (electrode area is about 16 cm ² , thermocompression bonding conditions) : 140 ° C. to 300 ° C., pressure 10 to 200 kg / cm ² , 10 seconds to 5 minutes).

<Production of PEMEC single cell>
The obtained MEA was set between two separators provided with flow paths to produce a PEMEC single cell (electrochemical cell).

  Using the produced single cell, the water electrolysis characteristics of the PEMEC cell and its stability were evaluated.

With respect to the obtained single cell, the cell temperature was maintained at 60 ° C., and water was supplied to the anode. A voltage of 1.3 to 2.5 V was applied to the single cell, and water electrolysis was performed for conditioning of MEA in about 5 hours. Thereafter, a voltage was applied to the single cell so that the current density was 2.5 A / cm ^2, and the voltage (V) after continuous water electrolysis for 1 hour was used as a voltage characteristic index for water electrolysis. 2.

For water electrolysis stability, PEMEC cell 10 minutes water electrolysis (supplying water to the anode, current density 2.5 A / cm ² ) and 10 minutes rest (switching water supply to the anode to nitrogen supply) The cycle was 2000 cycles. The voltage at the start of each cycle is the start voltage (V0), the voltage after the end of each cycle is the end voltage (Vf), and the voltage fluctuation rate ((V0−Vf) / V0) of each cycle is measured. As a stability index 1, an average voltage fluctuation rate of 2000 cycles was obtained. Further, the difference between the voltage after the first cycle (Vf1) and the voltage after 2000 cycles (Vf2000) was ΔV, and the deterioration rate (= ΔV / Vf1) was obtained as the stability index 2 of the PEMEC cell. The stability was evaluated according to the following criteria, and the evaluation results of each electrode are summarized in Tables 1 and 2.

Voltage fluctuation rate <7% A; Voltage fluctuation rate 8-15% B; Voltage fluctuation rate> 15% C; Degradation rate <7% A; Degradation rate 8-15% B; Degradation rate> 15% C;

  As shown in Tables 1 and 2, when compared with Comparative Examples 1 to 3, the MEAs of Examples 1 to 7 have a lower electrolysis voltage (V) required for the water electrolysis unit cell and good water electrolysis efficiency. Recognize. As for stability, both the voltage fluctuation rate and the deterioration rate are lower than those of the comparative example. The MEAs of Comparative Examples 1 and 2 are inferior in stability, and it is considered that there is no highly dispersed catalyst unit in the vicinity of the electrolyte membrane, or the degree of dispersion is insufficient. The MEA of Comparative Example 3 is considered to be due to a catalyst layer having a catalyst unit with a highly degraded water electrolysis characteristic and a low degree of dispersion.

According to at least one embodiment described above, an electrode having a highly dispersed catalyst unit in the vicinity of an electrolyte membrane can provide an electrode / membrane electrode assembly having high stability with a small amount of noble metal. At the same time, an electrochemical cell employing this electrode / membrane electrode complex can exhibit high stability and high durability.
In the specification, some elements are represented only by element symbols.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. Although the PEMEC single cell was mentioned as the electrochemical cell, the present invention can be similarly applied to other electrolytic cells. These novel embodiments described above can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Membrane electrode complex; 2 ... 1st electrode; 3 ... 2nd electrode; 4 ... Electrolyte membrane; 5 ... Catalyst layer; 5A ... 1st catalyst layer 5B ... 2nd catalyst layer 5C ... Intermediate layer; First gas diffusion layer; 7 ... catalyst layer; 8 ... second gas diffusion layer;

Claims (13)

A substrate;
A catalyst layer provided on the base material and including a plurality of catalyst units having a porous structure; and
The catalyst layer is provided in the vicinity of the base material, the first catalyst layer in which the catalyst units are dispersed at a first degree of dispersion, and the catalyst unit is dispersed at a second degree of dispersion. and a second catalyst layer and, Ri the second dispersion degree is different from the first dispersion degree,
The catalyst unit of the second catalyst layer is an electrode having an average distance between the catalyst units of 30 nm or more and 2000 nm or less .   The electrode according to claim 1, wherein the second degree of dispersion is higher than the first degree of dispersion.   The electrode according to claim 1, wherein the catalyst layer has an intermediate layer between the first catalyst layer and the second catalyst layer.   The electrode according to any one of claims 1 to 3, wherein the catalyst layer has a porosity of the catalyst layer of 50 vol% or more and 95 vol% or less. The average distance between the catalyst unit of the first catalyst layer, the electrode according to any one of claims 1 Ru 30nm less der than 0 nm 4.   The electrode according to any one of claims 1 to 5, wherein the catalyst unit of the first catalyst layer includes at least one of iridium oxide, ruthenium oxide, tantalum oxide, titanium oxide, platinum, and platinum oxide.   The electrode according to any one of claims 1 to 6, wherein an average height of the catalyst units of the second catalyst layer is 30 nm or more and 1000 nm or less.   8. The volume of 50 vol% or more of the catalyst units of the second catalyst layer is platinum, and the volume of 60 vol% or more of the catalyst units of the first catalyst layer is iridium oxide. Electrodes.   A membrane electrode assembly comprising the electrode according to any one of claims 1 to 8 and an electrolyte membrane.   The membrane electrode assembly according to claim 9, wherein the second catalyst layer is present between the first catalyst layer and the electrolyte membrane.   The membrane electrode assembly according to claim 9 or 10, wherein the electrolyte membrane is in physical contact with the second catalyst layer.   An electrochemical cell comprising the membrane electrode assembly according to any one of claims 9 to 11. A stack comprising the electrochemical cell according to claim 12.